« J'aurais eu un stylo. »
• Tense: conditionnel passé.
• Pronunciation: [oRԑ]
• Translation: I would have had a pen.
• Examples :
« Si vous l'aviez demandé, je vous aurais donné ce renseignement. »
« Si tu avais été prévoyant, tu aurais gardé un peu d'argent pour le bus. »
This tense (conditionnel passé) describes something that could have happened — but didn't. Your translation was correct : I would have had (...).
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« J'aurai eu un stylo. »
• Tense: futur antérieur.
• Pronunciation: [oRe]
• Translation: I will have had a pen.
• Examples :
« Tu partiras en voyage quand tu auras réussi tes examens. »
« Quand j'aurai vu un miracle, je croirai en Dieu. »
This tense (futur antérieur) describes a future event which takes place before another future event. Both happen in the future, but one happens before the other. Your translation was correct : I will have had (...).
Beware the pronunciation : [oRԑ] doesn't sound the same as [oRe].